Refactored #cd specs
#cd returns a ErrorENOENT on no dir
#preprocess_path uses #to_path on RemoteDirs
Added --irb option
#cat now returns an ErrorENOENT when the file doens't exist
Retry on #run errors only worked once; fixed
Added the stderr text to the exception
Added docs for builtins
Added #ls command for remote hosts
Migrated other old FSO methods